It is frequently desirable to measure the concentration of a substance such as oxygen, in a liquid or a gas. For example, it may be desirable to measure the amount of dissolved oxygen in a river for environmental purposes or in an industrial process to monitor the process.
A typical instrument presently used for such a measurement is the Clark cell which is described in U.S. Pat. No. 2,913,386 (Clark, Jr.). The Clark cell is an electrochemical cell in which electrodes are surrounded by an electrolyte. A membrane separates the electrolyte from a liquid containing the substance to be measured, and the membrane selectively passes the substance to the electrolyte. A voltage is then applied between the electrodes, and the amount of the current in the electrode circuit is an indication of the concentration of the substance to be measured in the electrolyte. This is in turn a measure of the concentration of the substance in the liquid under study.
The term "Clark Cell" when used hereafter refers to the polarographic implementation as well as the galvanic implementation.
In the polarographic implementation, the anode is usually of silver, and the cathode is usually either gold or platinum. The electrolyte typically is a potassium chloride solution at neutral pH, although alkaline or acid salt solutions are occasionally used. In this system, an external voltage, nominally 800 millivolts, is applied to the cell to drive the reaction.
In the galvanic implementation, a typical anode metal is lead. The cathode is generally platinum. When the electrolyte is a highly alkaline solution, such as potassium hydroxide having a pH greater than about 13, the anode rises to 800 millivolts, and no external voltage need be applied.
The Clark cell suffers from several disadvantages, one of which is the fouling of the membrane. U.S. Pat. No. 4,168,220 (McAdam, et al.) proposes a solution to this fouling problem by providing two distinct types of electrochemical cells adjacent each other. A comparison of the readings of the two cells is an indication of the extent to which the membrane is fouled.
Another, more significant, problem with the Clark cell is its consumption of the substance which is being measured. A common use of the Clark cell is to measure dissolved oxygen, and the oxygen passes from the liquid under study through a hydrodynamic boundary layer between the main body of the liquid and the membrane, through the membrane, and then into the electrolyte. Upon application of an appropriate voltage to the electrodes, the oxygen is electrochemically reduced at the negative electrode. This reduction depletes oxygen in the electrolyte and causes more oxygen to flow through the boundary layer and through the membrane from the liquid under study.
In the steady state, the rate of reduction equals the rate of oxygen flow through the membrane. The common practice in the use of the Clark cell is to consider the rate of reduction to be a measure of the amount of oxygen in the liquid. The boundary layer introduces an error, however, because it impedes the flow of oxygen through the membrane. The amount by which the boundary layer impedes the oxygen flow is a function of the thickness of the boundary layer and is thus unpredictable.
The extent to which the boundary layer exists is a function of the flow velocity of the liquid under study. If the liquid is stagnant, a substantial boundary layer will be produced, thus causing a significant error in the measurements.
One prior art solution is to artificially cause the liquid to move with respect to the cell to reduce the size of the boundary layer. This movement is caused, for example, by stirring the liquid which requires additional expense and results in an extremely complicated instrument.
A second technique relies upon modifying the characteristics of the Clark cell so that its rate of oxygen consumption per unit membrane area is diminished, which has the effect of reducing the influence of the boundary layer. This technique suffers from the disadvantage that the resulting instrument responds very slowly to changes in the concentration of dissolved oxygen and is in many instances essentially useless.